The present invention relates generally to a method of making a ceramic shell mould, particularly investment casting ceramic shell moulds and to a method of casting, particularly investment casting.
In investment casting, or lost wax casting process, a wax pattern of component, or article, is produced. The wax pattern is a replica of the component, or article, to be produced. Usually a number of wax patterns are assembled together on a wax runner to form a wax pattern assembly. Usually the wax runner is formed on a support member. The wax pattern assembly is immersed in a liquid ceramic slurry which quickly gels after draining, strengthening refractory granules are sprinkled over the ceramic slurry covered wax pattern assembly and the refractory granules bond to the slurry coating to produce a ceramic layer on the wax pattern assembly. This process is repeated several times to produce many ceramic layers which form the ceramic shell mould. The mould is dried and then heated so as to melt the wax pattern assembly such that the wax may run out of the mould to define an internal cavity identical in shape to the wax pattern assembly. The ceramic material is then fired to complete the ceramic shell mould. A suitable molten material, for example a metal, alloy, superalloy or intermetallic may then be cast in the ceramic shell mould.
It is well known that the investment casting process suffers a particular disadvantage in that when the ceramic coated wax pattern assembly is heated to remove the wax, the wax expands at a greater rate than the support member and ceramic shell mould. As a result of this differential thermal expansion, the ceramic shell mould sometimes cracks and this either renders the mould useless, or alternatively small fragments of the ceramic shell mould become entrained in the molten material and subsequently becomes lodged in the cast article and this may result in the article being deemed useless.
The use of waxes with lower melting points has been suggested as a way to overcome the problem, but this has not completely overcome the problem. The use of resilient portions in, or on, the wax runner portions as described in UK patent No. GB2068818A has also been suggested, but this does not completely overcome the problem.